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Apes adapted to metabolising ethanol ten million years ago

Alcohol has played a prominent role in human affairs throughout recorded history, but how far does its use go back? One view is that humans were teetotal until the advent of agriculture 9,000 years ago, when storage of food surpluses soon led to the invention of fermentation techniques. This model attributes the social problems associated with alcohol to the human metabolism not yet having had enough time to fully adapt to its consumption.

An alternative view is that primates became adapted to alcohol through eating fruit that was partially fermented through yeast infestation. Such fleshy fruits first appeared 80 million years ago, very early on in primate history and before the dinosaurs became extinct. The ‘evolutionary hangover’ model posits that arboreal primates foraging in trees became attracted to the smell of slightly-fermented fruit that might otherwise have gone unnoticed. This adaptation ceased to be beneficial once the attraction was turned to more strongly alcoholic drinks.

To resolve the issue, researchers resurrected digestive alcohol dehydrogenases (ADH4) enzymes from our primate ancestors to explore the history of primate interactions with ethanol. They reconstructed the evolutionary history of the ADH4 family of enzymes using ADH4 genes from 28 different mammals, including 17 primates. They synthesised nine of the ADH4 enzymes, from which they deduced that an ethanol-metabolising form was not yet appeared when orangutans diverged from gorillas, chimps and humans, but was present in the last common ancestor of gorillas, chimps and humans about ten million years ago.

The timing coincides with a shift to apes spending more time on the ground. It is likely that they began eating overripe, highly-fermented fruit that fell to the forest floor. At this time, apes faced growing competition from monkeys due to the ability of the latter to eat unripe fruit before it became suitable for consumption by apes. The ability to eat overripe fruit without becoming inebriated might have been an evolutionary adaptation to the problem.